Bamboo-like dual-phase nanostructured copper composite strengthened by amorphous boron framework

Grain boundary engineering is a versatile tool for strengthening materials by tuning the composition and bonding structure at the interface of neighboring crystallites, and this method holds special significance for materials composed of small nanograins where the ultimate strength is dominated by grain boundary instead of dislocation motion. Here, we report a large strengthening of a nanocolumnar copper film that comprises columnar nanograins embedded in a bamboo-like boron framework synthesized by magnetron sputtering co-deposition, reaching the high nanoindentation hardness of 10.8 GPa among copper alloys. The boron framework surrounding copper nanograins stabilizes and strengthens the nanocolumnar copper film under indentation, benefiting from the high strength of the amorphous boron framework and the constrained deformation of copper nanocolumns confined by the boron grain boundary. These findings open a new avenue for strengthening metals via construction of dual-phase nanocomposites comprising metal nanograins embedded in a strong and confining light-element grain boundary framework.

boron nano composite. Beside nano indentation extensive use of electron microscopy is made to identify the microstructure and changes due to plastic deformation by indentation. They found a very high hardness 10.8 GPa and explained this by 3 main processes: (i) indentation induces grain refinement, (ii) strong support of the boron network and (iii) enhanced stress response of the columnar copper grains due to the constrain of the boron network.
In the manuscript the authors mention often their material as copper alloy. In my opinion this is misleading as the structure of the current material is a nano composite consisting of columnar copper grains with a diameter of about 11 nm and an amorphous boron network at the "grain boundaries" with a thickness of about 2.5 nm. Hence a comparison with copper alloys is not useful. Overall, the structure and also the deformation processes looks similar to that of nanoscale multilayers. In fact, I am missing here some literature comparison of hardness and deformation patterning (e.g. bending of the layers).
The description of the deformation processes is a little bit speculative. All the mentioned processes make sense and it is likely that they contribute to the hardness increase, however, their contribution is not quantified or discussed which one is maybe the most important.  (2002)) shows that the nano hardness of amorphous boron thin films is between 30 and 35 GPa. If one assumes an area fraction of the amorphous boron of about 25 to 30% (see Fig. 1A top view where the indentation is performed or Fig. 1H) the measured hardness with 10.8 GPa can be explained by a simple parallel composite model. Hence, all the other discussed mechanisms are not necessary. A more in-depth discussion is needed here to justify the the proposed mechanisms and what contribution to hardness they really have. Also tensile properties (ductility) and fracture toughness would be of interest (I know this was not the scope of the current work, however, for a "good" material these parameters are also essential).
Overall, the manuscript presents some interesting work on the production of a Cu-B nano composite which can maybe extended to other material combinations. On the other side, the explanations of the so-called "record-high" hardness are insufficient. In my opinion, the hardness is not really "record high" and can be explained by the high hardness of the amorphous boron network by a simple composite model. Also, I am missing the interesting comparison with nanostructured multilayer thin films because some of the mentioned deformation mechanisms are also applying there. I think the manuscript doesn't meet the high standard of Nature Communication and I don't recommend publication.
1. EDS map for Cu-B coating has been provided. However, EDS line profiles should also be presented so that one can see the Cu composition modulation, especially across the amorphous phase boundaries.

Authors' reply:
We thank the reviewer for raising this issue. Following the reviewer's recommendation, we performed EDS and EELS examinations utilizing AC-STEM. We have added the following content in the revised image and high-loss image, respectively, of the selected locations in A. Point scanning is performed at position #1 within the TGB and at position #2 within the grain. (E) High-loss partial spectra at positions #1 and #2, where a more significant peak at around 200 eV is clearly observed at position #1 compared to position #2, which can be identified as the peak of element B 8 . (F) A magnified image of the energy loss around 200 eV, showing that the peak area at position #1 is ~6.2 times larger than that at position #2 after subtracting the background. This result demonstrates the enrichment of element B at the TGB." "8. Lu Y-G, Turner S, Ekimov EA, Verbeeck J, Van Tendeloo G. Boron-rich inclusions and boron distribution in HPHT polycrystalline superconducting diamond. Carbon 86, 156-162 (2015)." Comment 2: 2. It is known that residual stresses develop in sputtered films. The authors shall quantity residual stresses in these Cu-B alloy films.

3.
The elastic modulus of the Cu-B films should also be provided.

4.
Also have the authors looked at Cu-B alloy films with different B composition? A set of Cu-B films with variable composition will help to establish the solid trend showing the influence of B composition and grain size on mechanical behavior of the Cu-B coatings.
Authors' reply: We thank the reviewer for these comments. In response, we synthesized Cu-B thin films with varying B concentrations and characterized their composition, structure, and mechanical properties by XPS (Fig.   S1), XRD (Fig. S2), TEM (Figs. S8 and S9), and nanoindentation (Fig. 2), respectively. We have compiled the relevant information in Table S1 and added the following contents in the revised manuscript: "The addition of element B causes a significant decrease in the grain size, from 18.9 nm for the pure Cu film to 8.1 nm for the  at.% B film , 9.5 nm for  at.% B films, and 6.5 nm for .1 at.% B films. The grain size of Cu-B films does not show a regular pattern with increasing B concentration and is close to the critical grain size for Hall-Petch effect failure (Table S1)." (Line 124-127, Page 5).
"We have observed the cross-sectional morphology of the Cu-B films with varying B concentrations. The  at.% B film shows a morphology without significant features and there is a diffuse distribution of amorphous segregated phases (Fig. S8). Conversely, in the Cu-36.1 at.% B film, the fundamental columnar growth morphology endures, yet a discernible discontinuity exists in the columnar structure, and the progression of columnar grain growth becomes impeded by the amorphous B phase (Fig. S9)." (Line 127-132, Page 5).
"That is, the hardness of Cu-B films shows an increasing then decreasing trend with the increase of B concentration, while the trend of their moduli is consistent with the hardness (Table S1)." (Line 144-146, Page

5).
"We analyzed several factors that may affect the hardness of the films, including residual stresses and grain size.
Our measured results show that the films have small residual stresses which do not show a regular trend, indicating that the residual stresses have a negligible effect on the hardness. At the same time, Cu-B films with varying B concentrations have similar grain sizes, thus are not expected to have notably different contributions to hardness (Table S1)." (Line 148-152, Page 5).
We have added the following contents in the revised Supplementary Information. , where λ is the wavelength of the X-ray, β is the full width at half maximum (FWHM) of the peak, θ is diffraction angle, and k is a constant." " The hardness of Cu-B films with varying B concentrations is shown in Fig. 2A: Reviewer #2: The manuscript by Lv et al, "Nanostructured copper with amorphous boron decorated boundaries exhibits record-high hardness," suggests that the unique bamboo structure created by co-deposition using magnetron sputtering underlies the high hardness of this copper alloy. The work not only reports on the hardness, but also includes detailed STEM observation of the deformed material to relate the deformation mechanism to the structure. The work also includes ab initio molecular dynamics simulation to examine how the deposition process of Cu-B leads to the bamboo structure. Lastly, the authors perform DFT calculation for the solubility of B in Cu. In total, this is a very nice paper.

Authors' reply:
We appreciate the reviewer's positive appraisal of our work. nanolaminate structure with comparable layer thicknesses and compositions has significantly lower hardness.

Authors' reply:
We would like to express our gratitude to the reviewer for this valuable suggestion. We

Comment 2:
Lastly, the authors only provide information about the hardness, but very little regarding other mechanical properties, thermal stability, or potential applications.

Authors' reply:
We thank the reviewer for this suggestion. To provide the information on strength and ductility, we performed micropillar compression tests on the films (Fig. 6). The results show a yield strength of ~1.36 GPa and a flow stress of ~2.58 GPa, as well as a failure strain of over 50%. The thermal stability of the films is evaluated by measuring the structure and hardness of films after vacuum annealing at 200 ℃ for 1h (Fig. S11).
We have added the following contents to the revised manuscript: "Based on the previous analysis, it is speculated that the potential to restrict the shear behavior of the "bamboolike" dual-phase nanocomposite structure may impede material failure arising from shear deformation, thereby making a noteworthy contribution towards enhancing ductility. To verify this scenario, we conducted in-situ compression tests on the film, as displayed in the Supplementary Movie. The results indicate that the engineering stress-strain curve during the testing process is remarkably smooth and does not exhibit any pop-in points (Fig.   6A). On the premise that the deformation of the micropillar is uniform, the true stress-strain curve is derived and show a yield strength (σ0.2%) of ~1.64 GPa and a flow stress (σmax) of ~2.45 GPa (Fig. 6B). However, the deformation process of the micropillar is non-uniform, primarily dominated by barrel-shaped deformation at the top. Hence, by refitting the true stress-strain curve using the real-time measurement of the micropillar's crosssectional area (Fig. 6C The authors present a manuscript on the hardness / strength and deformation mechanisms of a copper -boron nano composite. Beside nano indentation extensive use of electron microscopy is made to identify the microstructure and changes due to plastic deformation by indentation. They found a very high hardness 10.8 GPa and explained this by 3 main processes: (i) indentation induces grain refinement, (ii) strong support of the boron network and (iii) enhanced stress response of the columnar copper grains due to the constrain of the boron network.
In the manuscript the authors mention often their material as copper alloy. In my opinion this is misleading as the structure of the current material is a nano composite consisting of columnar copper grains with a diameter of about 11 nm and an amorphous boron network at the "grain boundaries" with a thickness of about 2.5 nm. Hence a comparison with copper alloys is not useful. Overall, the structure and also the deformation processes looks similar to that of nanoscale multilayers. In fact, I am missing here some literature comparison of hardness and deformation patterning (e.g. bending of the layers).

Authors' reply:
We appreciate the valuable comments by the reviewer. As the reviewer pointed out, the "bamboolike" dual-phase Cu-B nanocomposite film we synthesized differs significantly from conventional alloy materials.
Therefore, we have deleted the expression "Cu-B alloy films" in the revised manuscript and changed it to "Cu-B films". We understand that alloying and constructing composite systems are both highly effective methods for enhancing the strength of metallic materials. Therefore, we compared the hardness of our synthesized film with In light of the similarity of the structure and deformation process to nanoscale multilayers that the reviewer mentioned, we synthesized a Cu/B multilayer film. The multilayer film is comprised of a crystalline Cu layer with a thickness of 10.6 nm and an amorphous B layer with a thickness of 3.6 nm, which has a similar elemental composition and structural dimensions to the "bamboo-like" Cu-B film (Fig. S10). Subsequently, we performed a nanoindentation test and observed the resulting structure beneath the indentation (Fig. S14). The results show that the Cu/B multilayer film we synthesized has distinct shear bands after deformation, which is consistent with the behaviors of the multilayers reported in the literature that exhibit layer bending phenomena. The deformation process is fundamentally different from that of the "bamboo-like" dual-phase Cu-B nanocomposite film presented in the present manuscript.
We have provided relevant descriptions in the revised manuscript: "…whereas the similar region of the Cu/B multilayer film does not produce any bending phenomenon and only a slight decrease in layer thickness (Figs. S13B and S13C)." (Line 181-182, Page 6).
"In contrast, the multilayer film sprouts a shear band across the film thickness in the region of larger deformation (Figs. S13F and S13G), with obvious faults on both sides of the shear band (Figs. S13H and S13I) but without the phenomenon of layer bending. It should be noted that nanoscale multilayers with layer bending during deformation are usually accompanied by shear bands 45,46,47,48,49 , and the deformation mechanism is significantly different from the column structure bending of the "bamboo-like" dual-phase Cu-B nanocomposite film." (Line

189-194 Page 6-7).
"In the region just below the indenter tip, the complex stress conditions produce significant grain refinement (Figs. 3N and 3O), producing a distinct GB morphology. IFFT images clearly distinguish random orientation changes of the (111)

Comment 2:
The description of the deformation processes is a little bit speculative. All the mentioned processes make sense and it is likely that they contribute to the hardness increase, however, their contribution is not quantified or discussed which one is maybe the most important.

Authors' reply:
We thank the reviewer for this comment. Quantifying the contribution of the deformation process to the hardness increase is a huge challenge. To clarify the contribution of the three main processes produced by the "bamboo-like" structure to the increase in hardness, we have performed additional experiments.
First, we synthesized Cu-B films with varying, both lower (10.3 at.%) and higher (36.1 at.%) B concentrations, and characterized their structures in detail. These Cu-B films have similar grain sizes; however, neither the higher nor the lower B concentration succeeds in building the "bamboo-like" dual-phase nanocomposite structure, and with lower hardness (Fig. 2) even when the B concentration is higher. After excluding the effect of residual stress on hardness, we find that the "bamboo-like" dual-phase Cu-B nanocomposite film exhibits the largest hardness compared with other Cu-B films. This result indicates that B content is not a main contributor to the hardness increase, implying a direct contribution of "enhanced stress response of the nanocolumnar copper structure constrained by the TGBs" to the hardness increase.
Furthermore, we have compared the deformation process of the Cu/B multilayer film with similar composition and structural dimensions with that of the "bamboo-like" dual-phase Cu-B nanocomposite film. The results show that, in addition to the "indentation induced grain refinement" which is common to both films, the "strong support of the TGBs consisting of an amorphous boron framework" and the "enhanced stress response of the nanocolumnar copper structure constrained by the TGBs" of the "bamboo-like" dual-phase Cu-B nanocomposite film contribute directly to the hardness increase.
Overall, the record-high hardness is achieved by the synergy of multiple mechanisms during the deformation process, which is caused by the specificity of the microstructure. Among these mechanisms, we believe that the most important one is the "Strengthening by enhanced stress response of constrained nanocolumnar copper".
Please refer to our responses to Reviewer #1's comment 2 and Reviewer #2's comment 1 for the pertinent new contents in the revised manuscript.
Also, the revised Fig. 2 has been added to the revised manuscript:

Authors' reply:
We appreciate the reviewer's suggestion, which inspired us to discuss the contribution of B concentration to the mechanical properties.
For this purpose, we synthesized a pure B film using the same B target under identical experimental parameters as used for the "bamboo-like" dual-phase Cu-B nanocomposite film, resulting in a measured hardness of approximately 19.6 GPa. We performed a rule of mixing (ROM) calculation in combination with the hardness of the pure Cu film, which predicted a much lower value than the "bamboo-like" dual-phase Cu-B nanocomposite film (Fig. 2). We have added the following content in the revised manuscript: "We performed nanoindentation tests on the synthesized films mentioned above in the continuous stiffness measurement (CSM) mode in order to determine their hardness ( Fig. 2A). The results indicate that the "bamboolike" dual-phase Cu-B nanocomposite film exhibits the largest hardness of 10.8 ± 0.3 GPa compared with other Cu-B films and exceeds the hardness of the Cu/B multilayer film. That is, the hardness of Cu-B films shows an increasing then decreasing trend with the increase of B concentration, while the trend of their moduli is consistent with the hardness (Table S1). Furthermore, the hardness of the Cu-B system is predicted using the rule of mixing (ROM), taking into account the hardness values of pure Cu and pure B films, while the "bamboo-like" dual-phase Cu-B nanocomposite film demonstrates considerably higher hardness than this prediction." (Line 141-148, Page

5)
We have added the following contents in the revised Supplementary  (G and H) The HRTEM images of the crystalline and the amorphous regions, respectively. The lattice stripes of both Cu(111) and Cu(200) orientations are present inside the crystalline region. The FFT image of the amorphous region is shown in the upper right corner, presenting a distinct amorphous halo."

Comment 4:
Also tensile properties (ductility) and fracture toughness would be of interest (I know this was not the scope of the current work, however, for a "good" material these parameters are also essential).
Authors' reply: This is a good suggestion. The hardness-ductility trade-off is a crucial issue that has long been faced by the field of structural materials, and it is a fundamental measurement of a "good" material that cannot be overlooked. To obtain a more comprehensive understanding of the mechanical properties of the films, we strived to conduct further tests on them. However, it is challenging to obtain tensile properties for film materials.
Conversely, the compression test provides a more widely applicable method for mechanical properties testing of thin film materials, facilitating a comparison with the existing literature. Therefore, we performed micropillar compression tests on the films to determine their strength and ductility. The results reveal a yield strength of ~1.36 GPa and a flow stress of ~2.58 GPa, as well as a failure strain of over 50%. These findings provide valuable insights into the overall mechanical behavior of the films.
The results of the micropillar compression tests are added to the revised manuscript as follows: "Based on the previous analysis, it is speculated that the potential to restrict the shear behavior of the "bamboolike" dual-phase nanocomposite structure may impede material failure arising from shear deformation, thereby making a noteworthy contribution towards enhancing ductility. To verify this scenario, we conducted in-situ compression tests on the film, as displayed in the Supplementary Movie. The results indicate that the engineering stress-strain curve during the testing process is remarkably smooth and does not exhibit any pop-in points (Fig.   6A). On this premise that the deformation of the micropillar is uniform, the true stress-strain curve is derived and show a yield strength (σ0.2%) of ~1.64 GPa and a flow stress (σmax) of ~2.45 GPa (Fig. 6B). However, the deformation process of the micropillar is non-uniform, primarily dominated by barrel-shaped deformation at the top. Hence, by refitting the true stress-strain curve using the real-time measurement of the micropillar's crosssectional area (Fig. 6C), we obtained a yield strength of ~1.36 GPa and a flow stress of ~2.58 GPa. Notably, the micropillar exhibited no shear bands or cracks even at a strain exceeding 50%, indicating high plasticity and ductility. In contrast, Cu-based micropillars reported in previous studies with similar strength did not exhibit such high ductility 56,57,58,59 . The results confirm our hypothesis that the "bamboo-like" dual-phase nanocomposite structure constrains the shear behavior, resulting in high strength and ductility. It signifies that the "bamboo-like" dual-phase Cu-B nanocomposite film is substantially hardened while retaining the intrinsic ductility of the metal, thereby achieving remarkable strengthening and toughening." (Line 280-295 Page 9-10). Furthermore, the SEM images of the micropillar at different strains are also provided in the figure."

Comment 5:
Overall, the manuscript presents some interesting work on the production of a Cu-B nano composite which can maybe extended to other material combinations. On the other side, the explanations of the so-called "record-high" hardness are insufficient. In my opinion, the hardness is not really "record high" and can be explained by the high hardness of the amorphous boron network by a simple composite model. Also, I am missing the interesting comparison with nanostructured multilayer thin films because some of the mentioned deformation mechanisms are also applying there.
I think the manuscript doesn't meet the high standard of Nature Communication and I don't recommend